1. Field of the Invention
This invention relates to acoustic speaker enclosures, and more particularly to a ported speaker enclosure in which the port is cylindrically wrapped completely around the enclosure thus maximizing port surface area and placing the long port in a more practical location.
2. Description of Related Art
Summary of definitions used herein:
B.sub.1 Product of magnetic flux density and wire length of coil (motor strength) PA1 f.sub.B Helmholtz resonance of vented or bandpass enclosure PA1 f.sub.h -3 dB lowpass cutoff of bandpass enclosure PA1 f.sub.1 -3 dB highpass cutoff of bandpass enclosure PA1 f.sub.3 -3 dB highpass cutoff frequency of closed or vented enclosures PA1 M.sub.t Total moving mass of driver PA1 P.sub.a Port surface area PA1 P.sub.l Port length PA1 P.sub.d Port diameter PA1 S.sub.d Effective surface area of driver PA1 V.sub.as Volume of air having same acoustic compliance as the driver suspension PA1 V.sub.d Peak displacement volume of cone PA1 V.sub.f Front chamber volume PA1 V.sub.r Rear chamber volume PA1 V.sub.t Total volume of chamber(s)
In the 1950s, the isobarik enclosure, as shown in FIG. 1, was first introduced. The isobarik enclosure loads multiple low frequency drivers into a sealed enclosure to effectively double M.sub.t and B.sub.1, while halving V.sub.as. An isobarik enclosure has several advantages over a sealed enclosure, as shown in FIG. 2. For example, it requires half the V.sub.t (with f.sub.3 held constant) or a lower f.sub.3 (with V.sub.t held constant). In either case, the isobarik system has the disadvantage of being less efficient (-3 dB).
Isobarik loading may also be used with ported enclosures, as shown in FIG. 3. Isobarik loading in a ported enclosure takes advantage of the smaller V.sub.t and lower f.sub.3 of an isobarik configuration, in addition to a lower f.sub.3 (compared to sealed enclosures) associated with ported enclosures, as shown in FIG. 4. Isobarik loading in a ported enclosure results in a smaller enclosure with superior frequency response. As with the isobarik enclosure discussed herein above, efficiency is compromised.
In addition, isobarik loading in a ported enclosure results in a problem associated with the port length. For a ported enclosure, if f.sub.3 is held constant and V.sub.t is reduced, then P.sub.l increases. However, an isobarik ported enclosure with half of V.sub.t will suffer an increase in P.sub.l ranging from two to three times that of a ported enclosure with the same f.sub.3. Port lengths of this magnitude are impractical for conventional cylindrical ports as P.sub.l approaches the largest enclosure dimension (length, width, or height).
Recent advances in motor design, cone materials, and adhesives have resulted in drivers with more than double the M.sub.t and B.sub.1 of conventional drivers. These new drivers effectively have a more massive cone and a stronger motor. Thus, their performance is similar to two conventional drivers in an isobarik configuration without the volume associated with the extra driver and joining chamber. Due to their small V.sub.t requirement, these drivers have unusually lengthy ports when used in ported enclosures. As with isobarik enclosures, efficiency is lower, but is compensated for by using high temperature materials and motor cooling techniques. Excursion capability is also increased, quadrupling the electrical and mechanical power handling of the driver. The net result is a speaker system with greater output and superior frequency response in a smaller enclosure. This new driver design is a trend arising from the demand for smaller, higher performance speaker systems.
Another recent advancement in motor technology has resulted in drivers with very high output capability. These drivers feature a more efficient motor system, better motor cooling, and greater excursion. Their electrical and mechanical power handling approaches one kilowatt RMS or four times that of many high performance drivers. These drivers can produce a wider range of frequencies at higher sound pressure levels when used in ported enclosures, making them ideal for professional sound reinforcement applications.
A quadrupling of electrical and mechanical power handling results in twice as much V.sub.d. For example, a driver may be capable of displacing a whopping 100 cubic inches of air, or twice that of a conventional driver with the same S.sub.d. To minimize non-linear port operation, P.sub.a must be doubled, increasing P.sub.d by a factor of approximately 1.4. An increase in P.sub.d of this magnitude will increase P.sub.l from two to four times if the f.sub.B and V.sub.t are held constant.
In summary, new technologies make it possible to produce low frequency speaker drivers with greater output (due to better power handling) and superior frequency response in a smaller enclosure. When used in ported enclosures, unusually long port lengths result that are four to twelve times longer given twice as much B.sub.1, M.sub.t, and V.sub.d. Such lengthy ports are often several feet in length, exceeding the largest enclosure dimension and rendering them very impractical. Consequently, ported designs using advanced, high output drivers need a better porting method that maximizes P.sub.a while reducing the impracticality of a lengthy P.sub.l.
Speaker system designers have already had to deal with fairly lengthy ports, even with conventional drivers, and have developed several modifications or alternatives to conventional cylindrical ports. A common solution is to flare (gradually widen) the port ends as shown in FIG. 5. A flare provides a smoother exit for the air as it escapes the port at high velocity. A flare in the port ends allows the designer to reduce P.sub.d by less than 40%. Reducing P.sub.d reduces P.sub.l but is not advisable since it also increases nonlinear port operation. Port flares are most often used to reduce port noise (caused by turbulence) of conventional cylindrical ports that already have an acceptable P.sub.d.
Passive radiators, as illustrated in FIG. 6, are sometimes used as well since they replace the port entirely through the use of a drone mass (usually a speaker cone and suspension without the motor structure). The drone has limited displacement compared to a port, requiring a very large diameter drone that can become impractical when using high output drivers.
Ducts are perhaps the most common way to implement unusually long port lengths, as shown in FIG. 7. Given a width to height ratio of less than 9:1, the length of a ducted port may be calculated in the same manner as a cylindrical port with the same P.sub.a. For an advanced, high output driver, the resulting duct would become a labyrinth as several feet of port length are shaped to the appropriate dimensions. Furthermore, designing a ducted enclosure is time consuming because any change in P.sub.a and P.sub.l requires that the labyrinth be redesigned. The construction of such an enclosure is surely a challenge.
There exists a need for a ported enclosure for high output loudspeakers that solve the hereinabove mentioned problems.